The present invention relates to dual energy x-ray imaging and measuring equipment that distinguish between multiple basis materials. Particularly, the invention is a scanning bone densitometry system that adjusts x-ray flux by varying the current level of the x-ray source.
The measurement of x-ray energy attenuated by an object in two distinct energy bands can be used to determine information about the materials of the imaged object. Generally attenuation is a function of x-ray energy according to two attenuation mechanisms of photoelectric absorption and Compton scattering. These two mechanisms differ among materials of different atomic numbers. For this reason, measurements at two energies can be used to distinguish between two different basis materials. Dual energy x-ray techniques can be used to separate bony tissue from soft tissue in medical imaging and quantitative bone density measuring.
In such dual energy measurements, the spectral characteristics of the x-ray source must be well known as it will affect the apparent relative attenuation of the patient for the two energies measured. It is typical to calibrate such densitometers for a given x-ray source operating at a given voltage and current.
The accuracy and precision of the images and measurements is a function of the amount of x-rays received by a radiation detector. Up to the point at which the detector becomes saturated, the more radiation received by the detector, the more precise the resulting images or measurements.
The amount of x-rays collected by the detector depends upon the amount of x-ray attenuation, which is a function of the attenuation by the patient and the duration of a sampling interval. The sampling interval is the time period during which x-rays are emitted and detected.
A number of methods have been developed to control the variation in x-ray flux received by the detector resulting from variations in patient tissue thickness and density, among patients or in a single patient over the course of a scan.
In one method, filters of different thicknesses are inserted into the radiation beam to provide greater attenuation for thinner patients and lesser attenuation for thicker patients. Such a system is described in a paper entitled X-Ray Bone Densitometry-New Developments by R. P. Nord et al. , and presented at the 21st European Symposium on Calcified Tissues, Jerusalem, Israel, March 1989.
An alternative approach controls the amount of x-ray flux received by the detector by varying the sampling time at the detector and hence the scan speed of the detector across the patient. Slower scan speeds are used for patients or portions of a patient having greater attenuation and higher scan speeds are used for patients or portions of a patient having lesser attenuation. The assignee of the present invention, Lunar Corporation used such a system in their DPX-IQ machine as disclosed in their operator manual for the DPX-IQ, pages 7.2 through 7.5.
Scan speeds may be preset or default settings used based on estimated patient attenuations. An important advantage to changing the scanning speed is that it controls the amount of x-ray flux received by the detector without changing the spectral characteristic of those x-rays such as may occur in the filter system.
Recent U.S. Pat. No. 5,687,211 to Noah Berger et al. discloses a variation on the technique of varying the scanning scan speed in which the scanner defaults to a fast scan speed and then drops back to slower scan speeds if the data collected indicates the patient thickness is unduly limiting the total x-ray flux.
A disadvantage to such systems is that they require a complex multi-speed scanning mechanisms and actuation devices. Also, as a rule such systems compromise scan speed by selecting the slowest scan speed required for any portion of the patient. The scan may be needlessly slow in portions of the scanned area or in areas where high precision is not required and lengthier scans create the risk of motion artifacts from involuntary or accidental patient muscle movement.
The dual energy scanning bone densitometry system of the present invention maintains a more closely optimized x-ray flux throughout an entire scan of the patient by adjusting x-ray current instead of or in addition to adjusting scan speed. The adjustment may be essentially continuous under computer control in which the thickness or location of the body region being scanned is determined to create a flux index. The computer increases or decreases x-ray current according to this flux index.
In a preferred embodiment, the dual energy scanning bone densitometry system uses a digital computer to execute a stored program to control actuators that move an x-ray tube and radiation detector so as to make successive scans across a patient. A computer controlled x-ray power supply provides input voltage and current to the x-ray tube. The x-ray tube emits x-rays that are attenuated by a region of a patient""s body and received by the radiation detector, which relays electrical signals to the digital computer. The computer uses the electrical signals from the detector to establish a flux index according to which it varies the x-ray current. The computer adjusts x-ray current if the flux index is not within pre-determined limits.
It is thus one object of the present invention to provide a mechanically simple method of controlling the amount of received x-ray signal.
It is another object of the invention to provide a method of controlling x-ray signals that may provide effectively continuous adjustment.
Bone density measurements in certain regions of the body, such as the lower lumbar vertebra and the upper portions of the femurs, may be more critical to the measurement of bone degeneration than in other regions. In another embodiment of the invention, the computer can identify these critical regions to increase the x-ray flux in these regions.
The adjustment process may be conducted for each scan line. Succeeding scan lines use the x-ray current settings of preceding scan lines until the scan is completed.
Thus, this invention may effectively provide continual flux adjustment, when needed, to maintain proper x-ray flux levels throughout an entire scan.
Thus, another object and advantage of the system of the present invention is to provide site-specific changes in x-ray flux to render more precise bone density date in regions of the body where such information is particularly critical.
The computer program adjusts the high and low energy x-ray signals according to any changes in x-ray spectrum caused by the adjustment of the x-ray current by use of a table containing empirically obtained correction factors at various x-ray current levels.
It is thus another object of the invention to provide this correction technique to maintain accurate dual energy data while altering flux through x-ray current adjustments.
In yet another embodiment, the densitometry system may adjust x-ray flux according to the flux index and body region by first adjusting x-ray current, and then, if the flux level remains unacceptable after adjusting the x-ray current to its limits, adjusting the speed of a multi-speed actuation system. In this embodiment, for example, a scan begins at a highest speed, and if the computer must increase x-ray current to a level higher than the maximum level in response to the desired flux index, then the computer signals the actuators to slow down. This increases the sample interval and thus the x-ray flux. This embodiment, therefore, provides additional flux adjustment capabilities for atypical patient thicknesses or heightened site-specific measurement precision. By adjusting current, and then speed only if necessary, the present invention provides a densitometry system that operates at the highest possible speed for as much of the scan as possible.
The foregoing and other objects and advantages will appear from the following description. In this description reference is made to the accompanying drawings which from a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.